PROJECT SUMMARY Multidrug-resistant (MDR) Plasmodium falciparum is largely responsible for the global resurgence of malaria. Artemisinin-based combination therapies (ACTs) are the most effective frontline treatments for Plasmodium falciparum malaria. Therefore, the recent emergence and spread of artemisinin resistance in the Greater Mekong Subregion of Southeast Asia, the epicenter of MDR P. falciparum, has raised global concerns. More worrisome, clinical resistance to dihydroartemisinin-piperaquine (DHA/PPQ) also has been detected recently in this region. Since DHA/PPQ is being considered for use in mass drug administration programs or for seasonal malarial prophylaxis in African settings, it is hence important to characterize the clinical phenotype of DHA/PPQ resistance and identify molecular markers of resistance to this drug. Artemisinin resistance, manifested as delayed parasite clearance following treatment with artemisinins, is associated with mutations in the K13 gene, but the molecular mechanism of K13-mediated artemisinin resistance remains to be elucidated. Moreover, the molecular mechanism of resistance to the partner drug piperaquine is not understood. To address this urgent problem, we will use a combination of molecular and population genomic approaches to investigate the mechanisms of antimalarial drug resistance and monitor the evolution and spread of resistant parasites. In this project, we propose to (1) identify molecular markers associated with piperaquine resistance, (2) determine the mechanisms of K13-mediated artemisinin resistance, and (3) longitudinally track the emergence and spread of artemisinin and piperaquine resistance in the strategically selected sentinel sites of the Greater Mekong Subregion. Results from this project will be highly useful for effectively monitoring, curbing and deterring the evolution of resistance to ACTs, and will provide guidance to malaria chemotherapy in and beyond this region.